Whistler instability driven by the sunward electron deficit in the solar wind

Abstract

Context. Solar wind electrons play an important role in the energy balance of the solar wind acceleration by carrying energy into interplanetary space in the form of electron heat flux. The heat flux is stored in the complex electron velocity distribution functions (VDFs) shaped by expansion, Coulomb collisions, and field-particle interactions. Aims. We investigate how the suprathermal electron deficit in the anti-strahl direction, which was recently discovered in the near-Sun solar wind, drives a kinetic instability and creates whistler waves with wave vectors that are quasi-parallel to the direction of the background magnetic field. Methods. We combine high-cadence measurements of electron pitch-angle distribution functions and electromagnetic waves provided by Solar Orbiter during its first orbit. Our case study is based on a burst-mode data interval from the Electrostatic Analyser System (SWA-EAS) at a distance of 112 R S (0.52 au) from the Sun, during which several whistler wave packets were detected by Solar Orbiter's Radio and Plasma Waves (RPW) instrument. Results. The sunward deficit creates kinetic conditions under which the quasi-parallel whistler wave is driven unstable. We directly test our predictions for the existence of these waves through solar wind observations. We find whistler waves that are quasi-parallel and almost circularly polarised, propagating away from the Sun, coinciding with a pronounced sunward deficit in the electron VDF. The cyclotron-resonance condition is fulfilled for electrons moving in the direction opposite to the direction of wave propagation, with energies corresponding to those associated with the sunward deficit. Conclusions. We conclude that the sunward deficit acts as a source of quasi-parallel whistler waves in the solar wind. The quasilinear diffusion of the resonant electrons tends to fill the deficit, leading to a reduction in the total electron heat flux.

Electron-driven instabilities in the solar wind

“…These energies are often comparable to the energy associated with the spacecraft electrostatic potential at the measurement point. Nevertheless, these modern observations confirm earlier suggestions that the electron distribution evolves with distance from the Sun and that non-thermal features are essential for a complete description of the evolution of the solar wind, especially near the Sun (Halekas et al, 2020;Berčič et al, 2021b;Halekas et al, 2021b;Abraham et al, 2022;Jeong et al, 2022b). These results and extrapolations based on previous measurements also suggest that electron-driven instabilities play an important role in the shaping of the electron distribution (Berčič et al, 2019), although many questions about electron kinetics and its impact on the evolution of the solar wind remain open.…”

“…High-cadence and high-resolution measurements of the electron distribution function from Solar Orbiter show pronounced deficits at times when pronounced amplitudes of quasi-parallel fastmagnetosonic/whistler waves are seen (Berčič et al, 2021b). This observation suggests the sporadic occurrence of the electron-deficit whistler instability in the solar wind.…”

“…( 16). The properties of this instability are still under study, but Berčič et al (2021b) suggest that the wave number at maximum growth is typically of order ∼ 1/d e .…”

Electron-Driven Instabilities in the Solar Wind

“…These energies are often comparable to the energy associated with the spacecraft electrostatic potential at the measurement point. Nevertheless, these modern observations confirm earlier suggestions that the electron distribution evolves with distance from the Sun and that non-thermal features are essential for a complete description of the evolution of the solar wind, especially near the Sun (Halekas et al, 2020;Berčič et al, 2021b;Halekas et al, 2021b;Abraham et al, 2022;Jeong et al, 2022b). These results and extrapolations based on previous measurements also suggest that electrondriven instabilities play an important role in the shaping of the electron distribution (Berčič et al, 2019), although many questions about electron kinetics and its impact on the evolution of the solar wind remain open.…”

The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons

“…Such a kinetic regime is difficult to treat with standard methods (e.g., Spitzer & Härm 1953;Gurevich & Istomin 1979). This is before consideration of instabilities that may affect the core (e.g., Schroeder et al 2021), which may even be generated by a resonant interaction with the deficit electrons themselves (Berčič et al 2021a). Thus the authors of B21 caution that their approach is "simplified and includes strong assumptions", and may cause either a systematic overestimation or underestimation of the potential depending on how the method is applied.…”